In recent years, the functionality of mobile digital electronic equipment, such as compact and thin digital AV players and digital cameras, has increased. High-speed non-volatile memory devices with greater capacity used as storage devices for such equipment are in increasing demand. In response to such a demand, much attention has been placed on non-volatile memory devices using ferroelectric capacitors and variable resistance elements that are kinds of non-volatile memory devices.
The variable resistance elements have two types: an only-once-writable type and a rewritable type. Furthermore, the rewritable variable resistance elements have two types. One type is variable resistance elements having characteristics that can reversibly change from a high resistance state to a low resistance state or from the low resistance state to the high resistance state, with application of two driving voltages of a same polarity. The variable resistance elements are generally called unipolar (or monopolar) variable resistance elements. The other type is variable resistance elements having characteristics that can reversibly change from the high resistance state to the low resistance state or from the low resistance state to the high resistance state, with application of two driving voltages of different polarities. The variable resistance elements are generally called bipolar variable resistance elements.
In the non-volatile memory device in which variable resistance elements are arranged in an array, the variable resistance elements are generally connected to current steering elements, such as transistors and rectifiers, thus preventing write disturbance caused by a bypass current in the array and crosstalk between adjacent memory cells. This structure secures the memory operation.
The unipolar variable resistance elements can control resistance change with two different driving voltages of the same polarity. Thus, unidirectional diodes using the nonlinear voltage-current characteristics of only one voltage polarity can be used as diodes functioning as current steering elements of the unipolar variable resistance elements, thus simplifying the structure of the memory cells each including a variable resistance element and the current steering element. However, the unipolar variable resistance elements need electric pulses with long pulse widths for reset operations for setting the variable resistance elements to the high resistance state, and thus the operating speed is low.
In contrast, the bipolar variable resistance elements control resistance change with two driving voltages of different polarities. Thus, the bipolar variable resistance elements need bidirectional diodes using the nonlinear voltage-current characteristics of the two voltage polarities as diodes functioning as current steering elements. The bipolar variable resistance elements can perform the reset operations for setting the variable resistance elements to the high resistance state and set operations for setting the variable resistance elements to the low resistance state, with application of electric pulses with shorter pulse widths at a high speed.
So far, PTL 1 suggests a cross point non-volatile memory device including memory cells in which variable resistance elements are connected in series with the unidirectional diodes functioning as current steering elements, such as p-n junction diodes and Schottky diodes.
Furthermore, PTL 2 suggests a cross point non-volatile memory device including memory cells in which variable resistance elements are connected in series with the bidirectional diodes as current steering elements.
For example, a metal-insulator-metal (MIM) diode, a metal-semiconductor-metal (MSM) diode, and a varistor disclosed in PTL 2 are known as the bidirectional diodes.
FIG. 16 illustrates voltage-current characteristics of a generally-known bidirectional diode. Such voltage-current characteristics are seen in the bidirectional diodes, such as an MIM diode, an MSM diode, and a varistor.
The voltage-current characteristics of such a bidirectional diode can be substantially symmetric with respect to the polarity of an applied voltage by optimizing materials of electrodes and materials to be interposed between the electrodes. In other words, it is possible to achieve the characteristics in which change in a current for a positive applied voltage and change in a current for a negative applied voltage substantially have a point symmetry with respect to the origin 0.
Furthermore, as illustrated in FIG. 16, the electric resistance of the bidirectional diode is very high when an applied voltage is between the second critical voltage Vth2 and the first critical voltage Vth1 inclusive (range C in FIG. 16), and sharply decreases when the applied voltage exceeds the first critical voltage Vth1 or falls below the second critical voltage Vth2 (ranges A and B in FIG. 16).
Combining the bidirectional diodes having such voltage-current characteristics with bipolar memory elements, that is, using the bidirectional diodes as current steering elements results in a cross point non-volatile memory device using the bipolar variable resistance elements.